This invention relates to a novel air heater design wherein compressed clean air is raised to elevated temperatures by heat supplied from the circulating fluidized bed combustion system, with the heated air leaving the air heater being expanded in a conventional gas turbine to drive an electric generator or other device. The air heater may be incorporated with equal effectiveness into a combined cycle or a cogeneration system, or a combination of the two.
The basic thermodynamic advantages of utilizing gas turbines in either combined cycle or cogeneration systems to generate electricity over the conventional electric utility central station installations are well established, and widely reported in the literature. The expanded use of gas turbines in these highly efficient cycles, however, has been limited because of the inability of gas turbines to accept the lower grade and more readily available fuels such as coal, petroleum coke, lignite, asphaltic residuum, etc. The current shortages of natural gas and liquid hydrocarbon fuels and their rapidly escalating costs have placed increased emphasis on the use of our large reserves of indigenous solid fuels in the generation of energy.
The present emphasis on the use of lower cost and more readily available solid fuels has initiated concerted efforts to develop gas turbine combined cycle and cogeneration systems wherein the primary energy source can be provided by solid or low grade liquid fuels such as asphaltic residuum. One effect of this effort is the development of the pressurized fluidized bed combustion system in which the products of combustion from the fluidized bed are directly expanded in a gas turbine, after cleanup, to provide the motive power for the gas turbine. This design appears to have a number of disadvantages including the following:
1. The difficulty and cost of gas cleanup at the elevated temperatures involved. PA1 2. The potential erosion on the turbine blades from the particulate matter which is not removed from the combustion gases. PA1 3. Potential fouling of the turbine from the particulate matter carried by the combustion gases. PA1 4. The relative high cost of a pressurized fluidized combustion system versus an atmospheric fluidized bed combustion system. PA1 1. By flowing across the tubes in cross-flow, higher air heat transfer film coefficients are developed at equal to, or lower frictional losses, than can be achieved with air flowing through the tubes. This results in significantly lower metal temperatures for the same air outlet temperature than can be attained in an air heater design having the air flowing through the tubes. PA1 2. The novel air heater design permits the use of extended surface on the outside of the tubes to enhance heat transfer and thus further reduce metal temperatures and permit the use of a lower tubular length to achieve the overall required heat transfer as compared to a conventional air heater with the air flowing through the tubes. PA1 3. The novel air heater results in a more compact and economical design than a conventional air heater having air flowing through the tubes. PA1 4. The novel air heater provides a design which is more accessible and easier to repair than a conventional air heater with the air flowing through the tubes.
A different approach to the development of solid or low grade liquid fired gas turbine combined cycle or cogeneration systems is to heat clean compressed air by heat generated in a fluidized bed combustor, and expand the clean heated air in a gas turbine. This arrangement permits the use of an essentially atmospheric pressure fluidized bed combustion system, and removes the problems associated with potential turbine blade erosion and turbine fouling anticipated with the pressurized fluidized bed combustion designs where the products of combustion are directly expanded in the gas turbine. All previous designs of air heaters proposed for use in the clean compressed air cycle feature tubular air heaters with the clean compressed air flowing through various tube configurations and heat being applied on the outside of the tubes.
A combined cycle power plant with atmospheric fluidized bed combustor is disclosed in U.S. Pat. No. 4,116,005 and one with a pressurized bed fluidized combustor in U.S. Pat. No. 4,223,529.
It is an object of our invention to provide an improved combined cycle or cogeneration power plant utilizing a coal-fired unpressurized fluidized bed combustor which supplies clean indirectly heated air to the gas turbine. It is another object to provide such a plant which utilizes a heat transfer unit external to its combustor to heat clean air for its gas turbines. It is another object to provide such a plant with an external heat transfer unit which operates with a lower pressure drop than has previously been obtainable.
It is another object of the invention to provide a novel air heater design which will provide greater reliability and service life than the conventional design having air flowing through the tubes of the air heater.
It is still another object of the invention to provide a novel air heater design which will operate at lower metal temperatures for the same outlet air temperature than a conventional air heater design having air flowing through the tubes.
A further object of the invention is to provide a novel air heater design which has lower frictional resistance to the flow of air than a conventional air heater having the air flowing through the tubes under identical performance conditions.
A still further object of the invention is to provide a novel air heater design which is more compact and more economical than a conventional air heater having the air flowing through the tubes under identical performance conditions.